Gas chromatography is often used in order to separate mixtures of substances, which can be subsequently analysed by means of mass spectrometry technique. As an example, according to the prior art, a GC-MS analysis apparatus for analysing mixtures of gaseous substances can comprise a sampling cell, or “sample loop” from which the gaseous mixture sample can be taken by a carrier gas; a sample valve adapted for sampling a precise volume of the gaseous mixture; a separation assembly and a detector.
The chromatographic column is subjected to a temperature variation schedule in order to achieve the separation of the components of the substances passing through the same column and analyzing when they come out from the column. For this reason the column is generally accommodated inside a controlled temperature oven. Nitrogen (N2), helium (He), or hydrogen (H2) can be used as a carrier gas.
The detector can be, for instance, a flame ionization detector (FID) associated to an electrometer converting the current collected by the FID in a voltage signal, which originates a chromatogram.
With reference to FIG. 1, a GC-MS analysis apparatus according to the prior art is schematically shown, which is provided with a gas chromatograph GC and a section MS. In the illustrated example, the section MS is equipped with an electronic impact (EI) ionic source and the analysis part is obtained by means of a quadrupole analyser. Other commonly used types of analyser can comprise magnetic sectors or an ionic trap.
An electronic impact (E.I.) ionic source (11), provided with an ionization filament (15), is associated to an outlet aperture (17) of a chromatographic column (19) into which the analyte is entered. The column (19) is usually accommodated inside a temperature controlled oven (25). An injector device (21) is provided at the inlet of the column (19) in order to bring the substance, which is to be analysed into the gas chromatograph and which has preliminarily rendered a solution by using an appropriate solvent.
The chromatographic column (19) generally consists of a capillary tube made of silica glass and typically having an inner diameter smaller than 1 mm, for example in a range between 220 and 250 μm, and a length higher than 10 m, for example in a range between 10 and 60 m.
A carrier gas (G1), for instance He, N2, H2, Argon is used for carrying into the column (19) the substance to be sampled. Other gases (G2), for instance methane (CH4), can be used for the chemical ionisation and sent to the ionic source (11) through an appropriate conduit (27). The analyte passes through the column (19), comes out from the aperture (17) and is ionised at the ionic source (11) accommodated in a first section S1 of the apparatus. At the exit of the ionic source (11) an ionic guide (35) is further provided, which is made of an electrostatic lens having the purpose to convey the ions to the subsequent section.
Downstream of the guide (35) a second ionic guide (37), for example a radio frequency hexapole or an octopole or miniquad, is generally positioned, through which the ionic beam is transmitted to a second section S2 of the apparatus, which is provided with a third ionic guide (39) and a quadrupole analyser (41). The sections S1 and S2 of the apparatus are accommodated inside a casing (23) and separated by a septum (46) having an orifice or “skimmer” (48).
The detector (43) is located downstream of the quadrupole (41) and is generally made of dynodes, namely electronic multipliers which are capable of amplifying the very low current produced by the ions which have passed the analyser. Examples of known detectors are Faraday cup detectors, SEM (Second Electron Multiplier) detectors and Channeltron detectors.
The described example relates to a “single quad”, however other devices and other quadrupoles can be provided along the path followed by the analyte and other quadrupoles, such as, for example, a collision reaction cell for removing interferences.
This kind of known apparatus is generally equipped with vacuum pumps (45, 47), for instance a pair of turbomolecular vacuum pumps, generally provided with corresponding mechanical “pre-vacuum” pump in order to generate vacuum conditions, for instance of 10−3 mbar (10−1 Pa) in the first section S1 and 10−5 mbar (10−3 Pa) in the second section S2 of the apparatus.
The GC-MS apparatuses are now broadly used in several fields of technology, but their use is becoming more and more widespread. Apparatuses of this kind are used for analysing volatile substances, e.g. for quantifying contaminants in the pharmacological and forensic field, and for analysing hazardous wastes, the quality of industrial products, the presence of organic pollutant in environmental samples, and the presence of undesired substances in the food. The known apparatuses are complicated and expensive to manufacture and to manage, and require remarkable investments. The supply of the carrier gas and other gases are added costs. Therefore, the need of having simplified GC-MS analysis apparatuses is highly felt.
Calibrated leak devices are also known in the art. Devices of this kind allow for generating controlled gas flows through the membrane as well as to quantificate leakages value by calibrating the instruments required to detect them during tight tests. The currently used devices are substantially of two kinds: orifice leaks, or capillary, and helium permeation leaks. The first ones, also called pinholes, are generally made by laser ablation or chemical etching. Such technologies enable apertures to be manufactured with high precision and reproducibility. An example of the first kind of devices having membranes with nanoholes, passing through the membrane and having a nanometric size diameter, is disclosed in the US patent publication no. 2006/0144120. Devices of this kind allows for generating controlled gas flows through the membrane as well as to quantificate leakages values, by calibrating the instruments required to detect them, during tight tests. Another example of this kind of membrane is disclosed in WO 03/049840.
The permeation leaks devices have, a very unstable behaviour when the temperature changes (their value varies of about 3% per centigrade grade in case of temperature values around room temperature), have long response times, are fragile (being made of glass, they are easily breakable even when they only fall to the ground), are only available for helium, and have a single flow value. Examples of such devices are described in DE 195 21 275 and WO 02/03057. Gas sampling devices based on permeation leaks are also disclosed in U.S. Pat. No. 4,008,388, US publication no. 2002/134933, U.S. Pat. No. 4,311,669, U.S. Pat. No. 4,712,008 and WO 2008/074984.
Selectively permeable membranes used in the field of mass spectrometry are also disclosed in U.S. Pat. No. 4,551,624 and Maden A M et Al: “Sheet materials for use as membranes in membrane introduction mass spectrometry” Anal. Chem., Am. Chem. Soc., US vol. 68, no. 10, 15 May 1996 (1996-05-15). Pages 1805-1811, XP000588711 ISSN:0003-2700.
Nanoholes membranes of the above first species have not to be confused with gas permeable membranes. Membranes of the first kind have holes made artificially, e.g. by laser drilling, having substantially regular cross section along the whole length of the hole and for this reason can be calibrated according to the use of the membrane; in addition, several or many almost identical holes with parallel axis can be produced on the same membrane. On the contrary, gas permeable membranes are membranes whose natural property of the material allows permeability of a gas or a gas mixture usually at a high temperature.
An object of the invention is to provide a simplified GC-MS apparatus, in particular with regard to the mechanical features and vacuum system, wherein the performances thereof are comparable with the ones of known apparatuses of the same category.
A further object of the invention is to provide an apparatus of the above specified type, which can be industrially mass produced with affordable costs.
Yet another object of the invention is to provide a GC-MS analysis apparatus, which is easier to manage with respect to the known ones having similar performances, and which therefore permits to reduce the maintenance costs.
These and other objects are achieved by means of a GC-MS apparatus as claimed in the appended claims.